1. Field of the Invention
The present invention relates to a zoom lens of two elements, a so-called retrofocus type, which comprises a front elements lens and a rear elements lens, wherein the front elements lens has a negative refractivity and the rear elements lens has a positive refractivity, and an image taking apparatus having the zoom lens as mentioned above.
2. Description of the Related Art
A zoom lens is classified broadly into a telephoto type having positive and negative refractive powers in sequence from the object side and a retrofocus type having negative and positive refractive powers in sequence from the object side. The two elements-retrofocus type of zoom lens has the following aspects as compared with the telephoto type.                (a) Fluctuations of F No. by zooming are small.        (b) Accuracy in a lens position is not severe.        (c) Peripheral light quantity at the wide-angle side is much.        
Further, the retro focus type of zoom lens has an aspect set forth below.                (d) Incident angle to an imaging plane is small, even if the wide-angle edge is concerned with.        
Particularly, with respect to the aspect of the item (d), it is a preferable advantage for a zoom lens, which is incorporated into a digital camera wherein solid state imaging devices such as CCD and MOS are arranged on an imaging plane. For this reason, there is a tendency that a digital camera adopts a retrofocus type of zoom lens.
On the other hand, the retrofocus type of zoom lens is associated with problems as set forth below.
According to the retrofocus type of zoom lens, of any one of lens elements after two elements, there exist lens elements (including direct magnification (lateral magnification: 1)), in which the lateral magnification for the infinite-object vary via the direct magnification, or extending over the low magnification less than the direct magnification and the high magnification exceeding the direct magnification, in accordance with the change of the focal length of the whole system by zooming. The lens elements including the direct magnification cannot be used as focusing lens elements at the area near the direct magnification or the direct magnification neighbor area of the zooming area.
Hereinafter, there will be explained the conventional example of a two elements-retrofocus type of zoom lens. In this example, the rear elements lens corresponds to lens elements including the direct magnification, since the two elements are concerned. Thus, it will be indicated that an arrangement of the rear elements makes it difficult to implement focusing in the direct magnification neighbor area.
FIG. 1 is an explanatory view of symbols used in the specification.
Here, lens elements illustrated at the left side of FIG. 1 are front elements 11 having a negative refractive power (the focal length is minus). And lens elements illustrated at the right side of FIG. 1 are rear elements 12 having a positive refractive power (the focal length is plus). Here, it is assumed that the “lens elements” include a case where the “lens elements” comprise only one piece of lens.
Here, there is assumed a digital camera in which a solid state imaging device such as a CCD imaging device is used to catch a subject image so as to generate image data. In FIG. 1, in addition to the zoom lens comprising the front elements 11 and the rear elements 12, there are shown a solid state imaging device 14 arranged in such a manner that a light receiving plane is coincident with an imaging plane 13, a cover glass 15 disposed in front of the light receiving plane of the solid state imaging device 14, and a low-pass filter 16 disposed in front of the cover glass 15. Further FIG. 1 shows an optical member having an effect on an optical path length, but omits mechanical shutter and an aperture member in illustration.
According to the digital camera, a subject light incoming via the zoom lens comprising the front elements 11 and the rear elements 12 is image-formed on the imaging plane 13 in which the light receiving plane of the solid state imaging device 14 is disposed, and the subject image formed on the imaging plane 13 is converted into an electric signal by the solid state imaging device 14.
Here, the symbols are defined as follows. Incidentally, it is noted that the definition is effective through the specification in its entirety. Further it is noted that the values are associated with the temperature variations, and the computing equations are computed using the associated values under the environment.
(a) A length from the top of the front elements on the optical axis to the front side main point of the front elements: Ha1 
(b) A main point interval of the front elements: HH1 
(c) A length from the rear side main point of the front elements to the front side main point of the rear elements where the air conversion is carried out on the optical member not included in the front elements and the rear elements: L
Where the length L is determined in a condition that the subject of the infinite-point is focused.
(d) A main point interval of the rear elements: HH2 
(e) A length from the top of the rear elements on the optical axis to the front side main point of the rear elements: Ha2 
(f) A focal length of the front elements: f1 (minus)
(g) A focal length of the rear elements: f2 (plus)
(h) A focal length of the whole system 10 comprising the front elements and the rear elements in zoom positions where the air conversion is carried out on the optical member interposed between the front elements and the rear elements: f (plus)
Incidentally, since the front elements and the rear elements are varied in relative length in accordance with the zoom position, the focal length f of the whole system 10 is a variable varying in accordance with the zoom position.
(i) A length from the subject at the time of near photography to the front side main point of the front elements where the air conversion is carried out on the optical member not included in the front elements: m
(j) A length from the subject at the time of near photography to the front side main point of the whole system where the air conversion is carried out on the optical member not included in the whole system: n
(k) A length from the top of the front elements on the optical axis to the imaging plane where the air conversion is carried out on the optical member not included in the front elements and the rear elements: T1 
(l) A length from the top of the rear elements on the optical axis to the imaging plane where the air conversion is carried out on the optical member not included in the rear elements: T2 
Here, the “length where the air conversion is carried out on the optical member not included in the front elements and the rear elements” denotes a “length” wherein when an optical member other than the front elements and the rear elements is disposed, taking into consideration the refractive index and thickness of the optical member, an optical length where a subject light passes through the optical member is converted into a length in air having the same optical length as the optical length where the subject light passes through the optical member.
For example, according to the example shown in FIG. 1, with respect to the length T1 in item (k), between the top of the front elements 11 on the optical axis, which is the starting point of the length T1 and the imaging plane 13, which is the terminal point of the length T1, as the optical member other than the front elements 11 and the rear elements 12, there are disposed the cover glass 15 and the low-pass filter 16. Taking into consideration the refractive index and thickness of the cover glass 15 and the low-pass filter 16, an optical pass length where a subject light passes through the cover glass 15 and the low-pass filter 16 is converted into a length in air having the same optical path length as the optical path length where the subject light passes through the cover glass 15 and the low-pass filter 16 so that the length T1 is determined.
That is, the length T1 after the air conversion, wherein the cover glass 15 and the low-pass filter 16 are disposed, is expressed by the formula as set forth below.
      T    1    =            T      0        +                                        n            1                    -          1                          n          1                    ⁢              t        1              +                                        n            2                    -          1                          n          2                    ⁢              t        2            
Where refractive index of the cover glass 15: n1, thickness of the cover glass 15: t1, refractive index of the low-pass filter 16: n2, thickness of the low-pass filter 16: t2, length T1 before an arrangement of the cover glass 15 and the low-pass filter 16: length T0 
In the event that the optical member other than the front elements 11 and the rear elements 12 is disposed between the rear elements 12 and the imaging plane 13, it is possible to convert the length T2 of the item (l) as well as the length T1of the item (k).
In a similar fashion to that, in the event that the optical member such as a crystal shutter, which has an effect on the optical path length, is disposed between the front elements 11 and the rear elements 12, the length L of the item (c), the focal length f of the item (h), and the length T1 of the item (k) are converted. Further, in a similar fashion to that, in the event that the optical member such as a cover glass for covering lens and an optical filter, which has an effect on the optical path length, is disposed before the front elements 11, the lengths m and n of the items (i) and (j) are converted, respectively.
With respect to the item (i), the definition “the length where the air conversion is carried out on the optical member not included in the front elements” is given. The reason why the word “rear elements” is omitted is that the rear elements do not exist between the subject and the front side main point of the front elements.
In a similar fashion to that, with respect to the focal length f of the whole system and the front side main point of the whole system, when there exists an optical member between the front elements 11 and the rear elements 12 as well as the front elements 11 and the rear elements 12, it is necessary to consider the optical path length of the optical member. On the other hand, the optical members before the front elements and after the rear elements have nothing to do with this. Thus, the “whole system” is defined, as expressed in the item (h), as the “whole system comprising the front elements and the rear elements in zoom positions where the air conversion is carried out on the optical member interposed between the front elements and the rear elements”.
Table 1 shows focal lengths of lens elements, main points and the like in the related art and the embodiments of the present invention.
TABLE 1Focal lengths of lens elements and main pointspositionsFrontMainRear sideFocalside mainpointmainlengthpointintervalspointFH1HHH2Front−12.42−1.090.75−5.44elementsRear10.26−1.101.62−4.07elements
Table 2 shows lens elements positions and focusing movement of the conventional zoom lens adopting the lens elements having the focal lengths of lens elements and main points positions shown in the table 1.
TABLE 2Lens elements positions and focusing movementof the conventional zoom lensZ1Z2z3z4z5z6z7z8z9z10Focal length(inf)5.706.206.787.458.249.1810.3311.7813.6516.13Magnification of−0.459−0.499−0.546−0.600−0.664−0.739−0.832−0.949−1.099−1.299rear elementsFront side main10.1210.029.909.779.619.439.249.098.147.71point of wholesystemFront element rear20.1918.3916.6314.9413.3011.7210.178.657.175.74side main point-rear element frontside main pointFront elements lens36.4435.0533.7732.6331.6530.8430.2429.9229.9930.60positionRear elements lens15.4815.9016.3816.9317.5818.3619.3120.5122.0524.10position (inf)Rear elements0.070.090.110.150.210.330.78focusFocus−0.67focusing movementnonnon(inf → 600 mm)
In table 2, Z1 to Z10 denote zoom stages. Here, there will be described 10 stages of zoom lens of Z1 to Z10. However, the explanation is applicable to not only the zoom lens in which the focal length is varied stepwise, but also the zoom lens in which the focal length is continuously varied.
FIG. 2 is a view showing positions of the front elements and the rear elements at the respective zoom stages of the zoom lens of the characteristics shown in Table 2.
FIG. 2 shows the top position of the front elements on the optical axis in the respective zoom stage wherein the imaging plane is established as the base, which top position of the front elements will be referred to simply as the “front elements position”, and the top position of the front elements on the optical axis, which top position of the rear elements will be referred to simply as the “rear elements position”.
With respect to the rear elements position, FIG. 2 shows both the position in which an image of the subject at the infinite-point is formed on the imaging plane, and the position in which an image of the subject at the closest distance (for example, 600 mm) is formed on the imaging plane.
Here, there is provided such a design that at the stage of zooming an image taking apparatus is focused on the subject appearing at the infinite-point (inf), and when the subject appears at an point closer than the infinite-point (inf), the rear elements are moved to a position according to the distance of the subject.
As known in general, in the event that the rear elements are moved to focus an image taking apparatus on the subject appearing at the near distance, the focusing is performed in such a manner that the rear elements are moved in a direction that the lateral magnification of the rear elements approaches the direct magnification (−1). As seen from table 2, in case of the zoom stages Z1 to Z7, the lateral magnification of the rear elements is lower magnification (−0.459 to −0.832) than the direct magnification (−1), and when the rear elements are moved in a direction that the rear elements approaches the front elements, the lateral magnification of the rear elements approaches the direct magnification (−1). Accordingly, in case of the zoom stages Z1 to Z7, when it is intended that the image taking apparatus is focused on the subject appearing at the near distance, the rear elements are moved in a direction that the rear elements approaches the front elements. On the other hand, in case of the zoom stage Z10, the lateral magnification of the rear elements is higher magnification (−1.299) than the direct magnification (−1), and when the rear elements are moved in a direction that the rear elements recedes from the front elements, the lateral magnification of the rear elements approaches the direct magnification (−1). Accordingly, in case of the zoom stage Z10, when it is intended that the image taking apparatus is focused on the subject appearing at the near distance, the rear elements are moved in a direction that the rear elements recedes from the front elements.
With respect to the zoom stages Z8 and Z9, when the rear elements are disposed at a position that an image taking apparatus is focused on the infinite-point (this is referred to as “disposed at the position of the infinite-point”), the lateral magnification of the rear elements already approaches the direct magnification (−1). As seen from table 2, in case of the zoom stage Z8, when the rear elements are disposed at the position of the infinite-point, the lateral magnification of the rear elements is −0.949, and in case of the zoom stage Z9, when the rear elements are disposed at the position of the infinite-point, the lateral magnification of the rear elements is −1.099.
In case of the zoom stage Z8, when the rear elements are disposed at the position of the infinite-point, the lateral magnification of the rear elements is −0.949. This is little lower magnification than the direct magnification. Thus, when an image taking apparatus is focused on the subject appearing at the near distance, the rear elements are moved in a direction that the rear elements approaches the front elements so that the lateral magnification of the rear elements approaches the direct magnification. However, since the lateral magnification of the rear elements disposed at the position of the infinite-point is near the direct magnification from the first, the image taking apparatus is not focused on the subject appearing at the nearest distance 600 mm, even if the rear elements are moved in a direction that the rear elements approaches the front elements, so that the lateral magnification of the rear elements is up to the direct magnification. In other word, in the zoom stage Z8, it is impossible to focus the image taking apparatus on the subject appearing at the nearest distance 600 mm.
This is applicable also to the zoom stage Z9. In case of the zoom stage Z9, when the rear elements are disposed at the position of the infinite-point, the lateral magnification of the rear elements is −1.099. This is little higher magnification than the direct magnification. Thus, when an image taking apparatus is focused on the subject appearing at the near distance, the rear elements are moved in a direction that the rear elements recede from the front elements, different from the case of the zoom stage Z8 in the direction, so that the lateral magnification of the rear elements approaches the direct magnification. However, in a similar fashion to that of the zoom stage Z8, since the lateral magnification of the rear elements disposed at the position of the infinite-point is near the direct magnification from the first, the image taking apparatus cannot be focused on the subject appearing at the nearer distance, by the reason that when the rear elements are moved in a direction that the rear elements recedes from the front elements, the lateral magnification of the rear elements is immediately the direct magnification.
It is recognized that the retrofocus type of zoom lens is associated with the above-mentioned defects. As one of measures, the zoom stages are set up in such a manner that zooming is impossible for an area in which focusing is impossible (for example, according to the example shown in FIG. 2, the zoom stages are set up in such a manner that the process jumps from the zoom stage Z7 to Z10 so as not to use the zoom stages Z8 and Z9). As other measures, Japanese Patent Application Laid Open Gazette TokuKai. 2001-281522 (FIG. 1) proposes a matter that an area for zooming is divided into a plurality of areas, and different lens elements are used as focusing lens elements for each divided area.
In Japanese Patent Application Laid Open Gazette TokuKai. 2001-281522, there is disclosed such a recognition that in case of the retrofocus type of zoom lens, lens elements including the direct magnification exist, and the lens elements including the direct magnification cannot be used as the focus lens elements. And there is disclosed an example of two elements-retrofocus type of zoom lens in which as to some area of the zooming areas the rear elements are moved to focus, and as to another area the front elements are moved to focus.
Recently, in a camera of type in which photography is carried out on a silver halide film, and particularly in a digital camera in which an image is formed on a solid state imaging device to generate image data, it is requested for those cameras to use a zoom lens and to provide compactness and thinness. In order to satisfy those requests, it is preferable to adopt two elements of zoom lens, which is less in the number of lens elements, in stead of three elements of zoom lens, which is conventionally used. However, the two elements-retrofocus type of zoom lens is associated with the problems explained referring to table 2 and FIG. 2. Thus, the conventional two elements-retrofocus type of zoom lens is an unsatisfactory zoom lens in specification, for example, there are taken measures that zooming is prohibited for the area wherein focusing is impossible. If only the focusing is considered, the use of the front elements for focusing makes it possible to focus. However, in general, the front elements are large in size, and are heavy in weight. Thus focusing of the front elements by it's movement with great accuracy needs a large type of motor having a large driving force. This causes the mechanical parts to be large, so that this is contrary to the requests of the compactness and the thinness, and further brings about the cost up.
Further, as proposed in Japanese Patent Application Laid Open Gazette TokuKai. 2001-281522, when it is intended that an area for zooming is divided into a plurality of areas, and different lens elements are used as focusing lens elements for each divided area, in case of two elements of zoom lens, it is obliged that large and heavy front elements are also used for focusing. Thus this also needs a large type of motor having a large driving force, and in addition needs a changeover mechanism for focus lens elements. This is contrary to the requests of the compactness, the thinness and the low cost.